With a view to further improving corrosion resistance of a zinciferous electro-galvanized steel sheet, trials are actively made to improve corrosion resistance of its zinciferous plating layer comprising zinc or a zinc alloy by uniformly dispersing oxide particles such as silica particles into the plating layer.
Japanese Patent Provisional Publication No.54-146,228 dated Nov. 15, 1979 discloses a composite zinc-silica electro-galvanized steel sheet having on at least one surface thereof a zinciferous plating layer into which silica particles are uniformly dispersed in an amount of from 2 to 15 wt. % (hereinafter referred to as "Prior Art 1").
The above-mentioned Prior Art 1 describes that the composite zinc-silica electro-galvanized steel sheet exhibits a corrosion resistance about 1.5 to 3 times as high as that of a conventional electro-galvanized steel sheet in a salt spray test.
Japanese Patent Provisional Publication No.61-143,597 dated July 1, 1986 discloses a composite zinc-silica electro-galvanized steel sheet having on at least one surface thereof a zinciferous plating layer, into which silica particles are uniformly dispersed in an amount of from 0.1 to 1.8 wt. % (hereinafter referred to as "Prior Art 2").
The above-mentioned Prior Art 2 describes that the composite zinc-silica electro-galvanized steel sheet shows a corrosion resistance about 4 to 8 times as high as that of a conventional electro-galvanized steel sheet in a salt spray test, and furthermore, the composite zinc-silica electro-galvanized steel sheet exhibits, if the surface of the plating layer thereof is treated by means of a silane coupling agent, a corrosion resistance more than about 20 times as high as that of the conventional electro-galvanized steel sheet.
Japanese Patent Provisional Publication No.60-141,898 dated July 26, 1985 discloses a composite zinc-oxide electro-galvanized steel sheet having on at least one surface thereof a zinc-alloy plating layer comprising zinc-nickel, zinc-iron or zinc-cobalt, into which particles of at least a kind of oxide such as silica are uniformly dispersed (hereinafter referred to as "Prior Art 3").
The above-mentioned Prior Art 3 describes that the composite zinc-oxide electro-galvanized steel sheet having a zinc-alloy plating layer comprising Zn-10wt. % Ni-10 wt. % SiO.sub.2 from among the above-mentioned zinc-alloy plating layers, has the highest corrosion resistance that is about twice as high as that of a conventional zinc-alloy electro-galvanized steel sheet having a zinc-alloy plating layer comprising Zn-13 wt. % Ni in a salt spray test.
As disclosed in the above-mentioned Prior Arts 1 to 3, it is possible to improve corrosion resistance of a zinciferous plating layer comprising zinc or a zinc alloy by uniformly dispersing silica particles therein, so as to further improve corrosion resistance of the zinciferous electro-galvanized steel sheet.
However, as described in Prior Arts 1 to 3, it is not easy to uniformly disperse silica particles into a zinciferous plating layer, because, like particles of the other oxides, silica particles are negatively charged in an electro-galvanizing bath and have a tendency of hardly precipitating onto the surface of a steel sheet serving as the cathode. For this reason, as described in Prior Art 1, dispersion of silica particles in a slight amount of only several wt. percent into a plating layer requires use of an electro-galvanizing bath containing silica particles at a high concentration of from 50 to 200 g/l. However, a high concentration of silica particles in the electro-galvanizing bath deteriorates stability of the electro-galvanizing bath, causes cohesion or precipitation of silica particles, and reduces the service life of the electro-galvanizing bath. In addition, electric resistance of the electro-galvanizing bath becomes unnecessarily higher during the electro-galvanizing, resulting in a poorer current efficiency of electro-galvanizing and an increased consumption of electric power for electro-galvanizing.
Japanese Patent Publication No.60-38,480 dated Aug. 31, 1985 discloses the use of silica particles subjected to a cation treatment for the purpose of facilitating dispersion of silica particles into a zinciferous plating layer (hereinafter referred to as "Prior Art 4").
It is true that the use of silica particles subjected to a cation treatment facilitates dispersion thereof into a zinciferous plating layer and increases the amount of silica particles in the plating layer. This is however is only due to the fact that the cation treatment helps silica particles migrate toward the plating layer on the surface of the steel sheet serving as the cathode in the electro-galvanizing bath, and hence, more silica particles are present near the plating layer. Therefore, there is no qualitative improvement in the dispersion of silica particles into the plating layer. More particularly, the distribution of silica particles throughout the plating layer is non-uniform, and this leads to an unstable corrosion resistance of the plating layer and a performance as desired cannot be achieved. In order to ensure achievement of a desired corrosion resistance of the plating layer, therefore, it is necessary to add more silica particles into the electro-galvanizing bath in an amount sufficient to make up for this possible loss in quality. This often results in a decreased service life of the electro-galvanizing bath due to the increase in concentration of silica particles, and an increased consumption of electric power due to the increased electric resistance of the electro-galvanizing bath.
The amount of the zinciferous plating layer, into which silica particles are dispersed, on at least one surface of the steel sheet, which is not specified in the above-mentioned Prior Arts 1 to 4, is usually within the range of from 5 to 100 g/m.sup.2 per side of the steel sheet.
Under such circumstances, there is a demand for the development of a composite zinc-silica electro-galvanized steel sheet excellent in corrosion resistance, having on at least one surface thereof a zinciferous plating layer with a high and stable corrosion resistance, into which silica particles are uniformly dispersed, but a composite zinc-silica electro-galvanized steel sheet provided with such a property has not as yet been proposed.